Method for producing ciliary marginal zone-like structure

ABSTRACT

The invention provides a method for producing a cell aggregate containing a ciliary marginal zone-like structure by culturing a cell aggregate containing a retinal tissue in which Chx10 positive cells are present in a proportion of 20% or more of the tissue in a serum-free medium or serum-containing medium, each containing a substance acting on the Wnt signal pathway for only a period before the appearance of a RPE65 gene expressing cell, followed by culturing the “cell aggregate in which a RPE65 gene expressing cell does not appear” thus obtained in a serum-free medium or serum-containing medium, each not containing a substance acting on the Wnt signal pathway and so on.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of copending U.S. patentapplication Ser. No. 14/406,440, filed Dec. 8, 2014, which is the U.S.national phase of International Patent Application No.PCT/JP2013/065878, filed Jun. 7, 2013, which claims the benefit ofJapanese Patent Application No. 2012-130521, filed Jun. 8, 2012, whichare incorporated by reference in their entireties herein.

TECHNICAL FIELD

The present invention relates to a method for producing a ciliarymarginal zone-like structure, and so on.

BACKGROUND ART

The ciliary marginal zone of the in vivo retina is known to performimportant functions for the structural formation and maintenance ofretinal tissues (see, for example, non-patent document 1) and, forexample, Rdh10 gene (non-patent document 2) and Otx1 gene (non-patentdocument 1) are known as gene markers of the ciliary marginal zone.However, there is no known method for producing such ciliary marginalzone-like structure from pluripotent stem cells with high efficiency.

DOCUMENT LIST Non-Patent Documents

non-patent document 1: W. Zac Stephens, Megan Senecal, Minhtu Nguyen,and Tatjana Piotrowski (2010) Loss of adenomatous polyposis coli (apc)Results in an Expanded Ciliary Marginal Zone in the Zebrafish Eye.DEVELOPMENTAL DYNAMICS Volume: 239, Pages: 2066-2077non-patent document 2: Fumi Kubo and Shinichi Nakagawa (2009) Hairylacts as a node downstream of Wnt signaling to maintain retinal stemcell-like progenitor cells in the chick ciliary marginal zone.Development Volume:136, Pages:1823-1833

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

There has been a desire to develop a method for producing a ciliarymarginal zone-like structure with high efficiency.

Means of Solving the Problems

The present inventors have conducted intensive studies in view of suchsituation and arrived at the present invention.

Specifically, the present invention provides:

-   1. a method for producing a cell aggregate comprising a ciliary    marginal zone-like structure, comprising a step of culturing a cell    aggregate comprising a retinal tissue in which Chx10 positive cells    are present in a proportion of 20% or more of the tissue in a    serum-free medium or serum-containing medium each containing a    substance acting on the Wnt signal pathway for only a period before    the appearance of a RPE65 gene expressing cell, followed by    culturing the “cell aggregate in which a RPE65 gene expressing cell    does not appear” thus obtained in a serum-free medium or    serum-containing medium each not containing a substance acting on    the Wnt signal pathway (hereinafter, sometimes referred to as the    production method of the present invention);-   2. the production method of the above-mentioned item 1, wherein the    period before the appearance of a RPE65 gene expressing cell is a    period during which Chx10 positive cells are present in the retinal    tissue in a proportion of within 50% to 1% of the tissue, and the    cell aggregate in which a RPE65 gene expressing cell does not appear    is a cell aggregate in which Chx10 positive cells are present in the    retinal tissue in a proportion of within 50% to 1% of the tissue;-   3. the production method of the above-mentioned item 2, wherein the    “cell aggregate in which a RPE65 gene expressing cell does not    appear” thus obtained is cultured in a serum-free medium or    serum-containing medium each not containing a substance acting on    the Wnt signal pathway until the proportion of Chx10 positive cells    present in the retinal tissue reaches 50% or more of the tissue;-   4. the production method of any of the above-mentioned items 1 to 3,    wherein the retinal tissue is derived from a human pluripotent stem    cell;-   5. use of a cell aggregate comprising a ciliary marginal zone-like    structure produced by the production method of any of the    above-mentioned items 1 to 4, as a reagent for evaluating toxicity    or drug efficacy;-   6. use of a cell aggregate comprising a ciliary marginal zone-like    structure produced by the production method of any of the    above-mentioned items 1 to 4, as a biological material for    transplantation;-   and so on.

Effect of the Invention

According to the production method of the present invention, a ciliarymarginal zone-like structure can be produced with high efficiency. In acell aggregate comprising a ciliary marginal zone-like structureproduced by the production method of the present invention, the ciliarymarginal zone-like structure functions as a progress zone, and there canbe formed with high frequency a continuous neural retina having a layerstructure adjacent to the ciliary marginal zone-like structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view that shows a GFP fluorescence image of Rax geneexpressing cells in a cryosection of cell aggregate before suspensionculture in a serum-free medium containing a substance acting on the Wntsignal pathway.

FIG. 2 is a view that shows a fluorescence immunostaining image, usingan anti-Chx10 antibody, of a cryosection of cell aggregate beforesuspension culture in a serum-free medium containing a substance actingon the Wnt signal pathway. Comparison of FIG. 1 (showing presence ofwhole retinal tissues) and FIG. 2 (showing presence of Chx10 positivecells) confirms presence of Chx10 positive cells in about 40% of thewhole before addition of a substance acting on the Wnt signal pathway.

FIG. 3 is a view that shows a GFP fluorescence image of Rax geneexpressing cells in a cryosection of cell aggregate on day 3 aftersuspension culture in a serum-free medium containing a substance actingon the Wnt signal pathway.

FIG. 4 is a view that shows a fluorescence immunostaining image, usingan anti-Chx10 antibody, of a cryosection of cell aggregate on day 3after suspension culture in a serum-free medium containing a substanceacting on the Wnt signal pathway. Comparison of FIG. 3 (showing presenceof whole retinal tissues) and FIG. 4 (showing presence of Chx10 positivecells) reveals presence of Chx10 positive cells only in about 3% of thewhole in 3 days after addition of a substance acting on the Wnt signalpathway to the medium, whereby a clear decrease can be confirmed.

FIG. 5 is a view that shows a GFP fluorescence image of Rax geneexpressing cells in a cryosection of cell aggregate after suspensionculture for 3 days in a serum-free medium containing a substance actingon the Wnt signal pathway followed by suspension culture for 39 days ina serum-containing medium not containing a substance acting on the Wntsignal pathway.

FIG. 6 is a view that shows a fluorescence immunostaining image, usingan anti-Rdh10 antibody, of a cryosection of cell aggregate aftersuspension culture for 3 days in a serum-free medium containing asubstance acting on the Wnt signal pathway followed by suspensionculture for 39 days in a serum-containing medium not containing asubstance acting on the Wnt signal pathway. Comparison of FIG. 5(showing presence of whole retinal tissues) and FIG. 6 (showing presenceof Rdh10 positive cells) confirms presence of Rdh10 positive cellsresulting from culturing for 3 days with the addition of a substanceacting on the Wnt signal pathway followed by culturing in aserum-containing medium not containing of a substance acting on the Wntsignal pathway.

FIG. 7 is a view that shows a GFP fluorescence image of Rax geneexpressing cells in a cryosection of cell aggregate after suspensionculture for 3 days in a serum-free medium containing a substance actingon the Wnt signal pathway followed by suspension culture for 50 days ina serum-containing medium not containing a substance acting on the Wntsignal pathway.

FIG. 8 is a view that shows a fluorescence immunostaining image, usingan anti-Rdh10 antibody, of a cryosection of cell aggregate aftersuspension culture for 3 days in a serum-free medium containing asubstance acting on the Wnt signal pathway followed by suspensionculture for 50 days in a serum-containing medium not containing asubstance acting on the Wnt signal pathway.

FIG. 9 is a view that shows a fluorescence immunostaining image, usingan anti-Otx1 antibody, of a cryosection of cell aggregate aftersuspension culture for 3 days in a serum-free medium containing asubstance acting on the Wnt signal pathway followed by suspensionculture for 50 days in a serum-containing medium not containing asubstance acting on the Wnt signal pathway. Comparison of FIG. 7(showing presence of whole retinal tissues), FIG. 8 (showing presence ofRdh10 positive cells) and FIG. 9 (showing presence of Otx1 positivecells) confirms presence of Rdh10 positive cells and Otx1 positive cellresulting from culturing for 3 days with the addition of a substanceacting on the Wnt signal pathway followed by culturing in aserum-containing medium not containing of a substance acting on the Wntsignal pathway.

FIG. 10 shows a staining image of a cryosection of a region containing aciliary marginal zone-like structure contained in a cell aggregateprepared as shown below. A cell aggregate on day 18 after the start ofsuspension culture was suspension cultured for 3 days in a serum-freemedium containing a substance acting on the Wnt signal pathway andsuspension cultured for 46 days in a serum-containing medium notcontaining a substance acting on the Wnt signal pathway, then culturedfor 1 day in the presence of BrdU, then cultured for 13 days in theabsence of BrdU, and cultured for 1 day in the presence of EdU(Invitrogen). Cryosections of the obtained cell aggregate were preparedand subjected to fluorescence immunostaining with an anti-Ki67 antibody(left Figure) or an anti-BrdU antibody (right Figure), or colordevelopment reaction with EdU (middle Figure).

FIG. 11 shows a staining image of a cryosection of a region containing aciliary marginal zone-like structure contained in a cell aggregateprepared as shown below. A cell aggregate on day 18 after the start ofsuspension culture was suspension cultured for 3 days in a serum-freemedium containing a substance acting on the Wnt signal pathway andsuspension cultured for 46 days in a serum-containing medium notcontaining a substance acting on the Wnt signal pathway, then culturedfor 1 day in the presence of BrdU, then cultured for 13 days in theabsence of BrdU, cultured for 1 day in the presence of EdU (Invitrogen),and further cultured for 13 days in the absence of EdU. Cryosections ofthe obtained cell aggregate were prepared and subjected to fluorescenceimmunostaining with an anti-Ki67 antibody (left Figure) or an anti-BrdUantibody (right Figure) or an anti-Rdh10 antibody (lower Figure), orcolor development reaction with EdU (middle Figure).

FIG. 12 is a view that shows analysis of a cell aggregate obtained bysubjecting a cell aggregate on day 18 after the start of suspensionculture to suspension culture for 3 days in a serum-free mediumcontaining a substance acting on the Wnt signal pathway and suspensionculture for 75 days in a serum-containing medium not containing asubstance acting on the Wnt signal pathway.

FIG. 12A is an example of a cell aggregate under said conditions, andshows a phase contrast image of a cell aggregate without a ciliarymarginal zone-like structure (CMZ) (A, left row, upper panel), a GFPfluorescence image of Crx gene expressing cells in a cell aggregatewithout a ciliary marginal zone-like structure (A, left row, lowerpanel), a phase contrast image of a cell aggregate containing a ciliarymarginal zone-like structure (A, right row, upper panel), and a GFPfluorescence image of a Crx gene expressing cell in a cell aggregatecontaining a ciliary marginal zone-like structure (A, right row, lowerpanel).

FIG. 12B is a graph relating to cell aggregates without a ciliarymarginal zone-like structure (CMZ−) and cell aggregates with a ciliarymarginal zone-like structure (CMZ+), which shows the measurement resultsof the proportion of cell aggregates containing continuous, stratifiedneural retina in not less than 10% of the circumference of the cellaggregate, by using the form of the Crx gene expressing cell as anindex.

MODE(S) FOR CARRYING OUT THE INVENTION

Mode(s) for carrying out the present invention is explained in detailbelow.

In the present invention, examples of the “stem cell” include a cellthat maintains the same differentiation capacity even after celldivision and can regenerate a tissue when it is injured. Here, the “stemcell” may be an embryonic stem cell (ES cell) or a tissue stem cell(also called tissular stem cell, tissue-specific stem cell or somaticstem cell), or an artificial pluripotent stem cell (iPS cell: inducedpluripotent stem cell) but is not limited thereto. As is appreciatedfrom the fact that the stem cell-derived tissue cell can regenerate atissue, it is known that the stem cell can differentiate into a normalcell close to one in a living body.

Examples of the “pluripotent stem cell” in the present invention includea stem cell that can be cultured in vitro and has an ability todifferentiate into any cell (triploblast (ectoderm, mesoderm,endoderm)-derived tissue) constituting a living body except for placenta(pluripotency), including an embryonic stem cell (ES cell). The“pluripotent stem cell” is obtained from fertilized egg, clone embryo,reproductive stem cell, and stem cell in a tissue. It also includes acell having artificial pluripotency similar to that of embryonic stemcells, after introducing several kinds of genes into a somatic cell(also called artificial pluripotent stem cell). Pluripotent stem cellcan be produced by a method known per se. Examples of the productionmethod include the methods described in Cell 131(5) pp. 861-872 (2007),Cell 126(4) pp. 663-676 (2006), etc.

Examples of the “embryonic stem cell (ES cell)” in the present inventioninclude a stem cell having a self replication ability and multipotency(i.e., “pluripotency”), which is a pluripotent stem cell derived from anearly embryo. Embryonic stem cell was first established in 1981, and hasalso been applied to the generation of knockout mouse since 1989. In1998, a human embryonic stem cell was established, which is also beingutilized for regenerative medicine.

Examples of the “artificial pluripotent stem cell” in the presentinvention include a cell induced to have multipotency by directlyreprogramming a differentiated cell such as fibroblast etc. by theexpression of several kinds of genes such as Oct3/4, Sox2, Klf4, andMyc, which was established by Yamanaka et al. in mouse cell in 2006(Takahashi K, Yamanaka S. Cell. 2006, 126(4), p 663-676). In 2007, theartificial pluripotent stem cell was also established in humanfibroblast, and has multipotency similar to that of embryonic stem cells(Takahashi K, Tanabe K, Ohnuki M, Narita M, Ichisaka T, Tomoda K,Yamanaka S. Cell. 2007, 131(5), p 861-872.; Yu J, Vodyanik M A,Smuga-Otto K, Antosiewicz-Bourget J, Frane J L, Tian S, Nie J,Jonsdottir G A, Ruotti V, Stewart R, Slukvin I I, Thomson J A., Science.2007, 318(5858), p 1917-1920.; Nakagawa M, Koyanagi M, Tanabe K,Takahashi K, Ichisaka T, Aoi T, Okita K, Mochiduki Y, Takizawa N,Yamanaka S. Nat Biotechnol., 2008, 26(1), p 101-106).

Pluripotent stem cells are available from given organizations, or acommercially available product can be purchased. For example, humanembryonic stem cells, KhES-1, KhES-2 and KhES-3, are available fromKyoto University's Institute for Frontier Medical Sciences. EB5 cell,which is a mouse embryonic stem cell, is available from RIKEN, and D3cell line is available from ATCC, respectively.

Pluripotent stem cell can be maintained by culturing according to amethod known per se. For example, human stem cell can be maintained byculturing using Knockout Serum Replacement (KSR). For example, mousestem cell can be maintained by culturing with addition of fetal bovineserum (FCS) and LIF, and without feeder cell.

Examples of the “tissue” in the present invention include a structure ofa cell population, which has a conformation wherein more than one typeof cell different in the shape and property are sterically configured ina given pattern.

In the present invention, examples of the “retinal tissue” include aretinal tissue etc. wherein at least two or more types of cells such asphotoreceptors, horizontal cells, bipolar cells, amacrin cells, retinalganglion cells, their precursor cells, retinal progenitor cells thereofand so on, which constitute respective retinal layers in in vivo retina,are sterically arranged in layers. With regard to each cell, which cellconstitutes which retinal layer can be confirmed by a known method, forexample, presence or absence of the expression of a cell marker or thelevel thereof, etc.

Examples of the retina cell marker include Rax (progenitor cell ofretina), PAX6 (progenitor cell), Chx10 (neural retinal progenitor cell),nestin (expressed in progenitor cell of hypothalamus neuron but notexpressed in retinal progenitor cell), Sox1 (expressed in hypothalamusneuroepithelium but not expressed in retina), Crx (precursor cell ofphotoreceptor), and so on. Examples of the marker of the above-mentionedretinal layer-specific neuron include Chx10 (neural retinal precursorcell or bipolar cell), L7 (bipolar cell), Tuj1 (ganglion cell), Brn3(ganglion cell), Calretinin (amacrine cell), Calbindin (horizontalcell), Rhodopsin (photoreceptor), Recoverin (photoreceptor), RPE65(pigment epithelium), Mitf (pigment epithelium) Nrl (rod cell),Rxr-gamma (cone cell) and so on.

The “retinal layer” in the present invention means each layerconstituting the retina. Specific examples thereof include retinalpigment epithelial layer, photoreceptor layer, external limitingmembrane, outer nuclear layer, outer plexiform layer, inner nuclearlayer, inner plexiform layer, ganglion cell layer, nerve fiber layer andinner limiting membrane.

Examples of the “ciliary marginal zone (CMZ)” in the present inventioninclude a tissue present in the boundary region of retinal tissue(specifically, neural retina) and retinal pigment epithelium in the invivo retina, which is a region including a tissue stem cell of retina(retinal stem cell). Examples of the marker gene of the ciliary marginalzone include Rdh10 gene (positive), Otx1 gene (positive) and so on. Itis known that the ciliary marginal zone plays an important role in thesupply of retinal progenitor cells and differentiated cells to retinaltissues, maintenance of retinal tissue structure and so on.

Examples of the “progress zone” in the present invention include apopulation of undifferentiated cells localized in a part of a tissue,and examples thereof include a population of cells having properties tocontinuously grow in the process of development and regeneration tocontribute to the growth of a tissue as a whole and/or properties tocontribute to the growth of the surrounding tissues by secreting agrowth factor etc . . . Specific examples of the progress zone include apopulation of undifferentiated cells at the tip of a limb bud.

Examples of the “aggregate” in the present invention include a mass ofthe cells dispersed in the medium but gathered to form same. The“aggregate” in the present invention includes an aggregate formed by thecells dispersed at the start of the suspension culture and an aggregatealready formed at the start of the suspension culture.

When cells gather to form cell aggregates and the aggregates aresubjected to suspension culture, to “form aggregate” means to “rapidlyaggregate a given number of dispersed stem cells” to form qualitativelyuniform cell aggregates.

Examples of the experimental operation to form an aggregate include amethod involving keeping cells in a small space by using a plate withsmall wells (96 well plate), micropore and so on, a method involvingaggregating cells by centrifugation for a short time using a smallcentrifugation tube, and so on.

The “medium” to be used in the present invention can be prepared from amedium used for culture of animal cell as a basal medium. Examples ofthe basal medium include media that can be used for culturing animalcells such as BME medium, BGJb medium, CMRL1066 medium, Glasgow MEMmedium, Improved MEM Zinc Option medium, IMDM medium, Medium199 medium,Eagle MEM medium, αMEM medium, DMEM medium, ham medium, RPMI1640 medium,Fischer's medium, and mixed medium thereof etc.

Examples of the “serum-free medium” in the present invention include amedium free of unadjusted or unpurified serum. In the present invention,a medium containing purified blood-derived components and animaltissue-derived components (e.g., growth factor) is also included in aserum-free medium unless unadjusted or unpurified serum is containedtherein.

To avoid complicated preparation, a serum-free medium (GMEM or DMEM, 0.1mM 2-mercaptoethanol, 0.1 mM non-essential amino acid Mix, 1 mM sodiumpyruvate) added with an appropriate amount (e.g., 1-20%) of commerciallyavailable KSR can be preferably mentioned as the serum-free medium.

In addition, the serum-free medium may contain a serum replacement.Examples of the serum replacement include albumin, transferrin, fattyacid, collagen precursor, trace element, 2-mercaptoethanol or 3′thiolglycerol, one appropriately containing equivalents of these etc.,and so on. Such serum replacement may be prepared by, for example, themethod described in WO98/30679 and so on. In addition, the serumreplacement may be a commercially available product. Examples of suchcommercially available serum replacement include Chemically-definedLipid concentrated (manufactured by Gibco), Glutamax (manufactured byGibco) and so on.

The “serum-free medium” to be used for suspension culture may containfatty acid, lipid, amino acid (e.g., non-essential amino acid), vitamin,growth factor, cytokine, antioxidant, 2-mercaptoethanol, pyruvic acid,buffering agent, inorganic salts and so on.

Examples of the “serum-containing medium” in the present inventioninclude a medium containing unadjusted or unpurified serum. The mediummay contain fatty acid, lipid, amino acid (e.g., non-essential aminoacid), vitamin, growth factor, cytokine, antioxidant, 2-mercaptoethanol,pyruvic acid, buffering agent, inorganic salts and so on.

Examples of the “suspension culture” in the present invention includeculture of cell aggregates in a medium under non-adhesive conditions toa cell culture vessel, and so on.

The cell culture vessel to be used in suspension culture is notparticularly limited as long as it enables suspension culture of thecells. Examples of such cell culture vessel include flask, tissueculture flask, dish, petri dish, tissue culture dish, multidish,microplate, microwell plate, micropore, multiplate, multiwell plate,chamber slide, schale, tube, tray, culture bag, roller bottle and so on.A preferable vessel is a cell non-adhesive vessel.

As a cell non-adhesive vessel, one having its surface not artificiallytreated to improve cell adhesiveness (e.g., coating treatment withextracellular matrix, etc.) and so on may be used.

Examples of the “serum” to be added to the medium in the presentinvention include mammalian sera such as bovine serum, calf serum, fetalbovine serum, horse serum, colt serum, fetal horse serum, rabbit serum,leveret serum, fetal rabbit serum, and human serum, and so on.

The production method of the present invention characteristicallyincludes a step of culturing a cell aggregate comprising a retinaltissue in which Chx10 positive cells are present in a proportion of 20%or more of the tissue in a serum-free medium or serum-containing mediumeach containing a substance acting on the Wnt signal pathway for only aperiod before the appearance of a RPE65 gene expressing cell, followedby culturing the “cell aggregate in which a RPE65 gene expressing celldoes not appear” thus obtained in a serum-free medium orserum-containing medium each not containing a substance acting on theWnt signal pathway. The “cell aggregate comprising a ciliary marginalzone-like structure” produced by the production method of the presentinvention is useful as a reagent for use for the evaluation of toxicityor drug efficacy of chemical substances and so on, or a material for usefor the tests or treatments aiming at cell therapy and so on.

The “cell aggregate comprising a retinal tissue” to be used as astarting material in the production method of the present invention is acell aggregate in which Chx10 positive cells are present in the retinaltissue in a proportion of 20% or more of the tissue. The aforementioned“proportion of Chx10 positive cells” is, for example, preferably notless than 40%, more preferably not less than 60%, particularlypreferably not less than 80%.

The “cell aggregate comprising a retinal tissue” to be used as astarting material in the production method of the present invention canbe prepared, for example, from a pluripotent stem cell (preferably humanpluripotent stem cell). Specifically, for example, it can be prepared bya method including the following steps (1) to (3).

-   (1) a first step of subjecting pluripotent stem cells to suspension    culture in a serum-free medium containing a substance inhibiting the    Wnt signal pathway to form an aggregate of pluripotent stem cells,-   (2) a second step of subjecting the aggregate formed in the first    step to suspension culture in a serum-free medium containing a    basement membrane preparation, and-   (3) a third step of subjecting the aggregate cultured in the second    step to suspension culture in a serum-containing medium.

A substance inhibiting the Wnt signal pathway to be used in the firststep is not particularly limited as long as it can suppress signaltransduction mediated by Wnt. Examples of the substance inhibiting theWnt signal pathway include Dkk1, Cerberus protein, Wnt receptorinhibitor, soluble-type Wnt receptor, Wnt antibody, casein kinaseinhibitor, dominant negative Wnt protein, CKI-7(N-(2-aminoethyl)-5-chloro-isoquinoline-8-sulfonamide), D4476(4-{4-(2,3-dihydrobenzo[1,4]dioxin-6-yl)-5-pyridin-2-yl-1H-imidazol-2-yl}benzamide),IWR-1-endo (IWR1e), IWP-2 and so on. The concentration of the substanceinhibiting the Wnt signal pathway only needs to be a concentration atwhich aggregates of pluripotent stem cells are formed. For example, acommon substance inhibiting the Wnt signal pathway such as IWR1e isadded at a concentration of about 0.1 μM to 100 μM, preferably about 1μM to 10 μM, more preferably about 3 μM.

A substance inhibiting the Wnt signal pathway may be added to serum-freemedium before the start of the suspension culture, or added to aserum-free medium within several days from the start of the suspensionculture (e.g., within 5 days). Preferably, a substance inhibiting theWnt signal pathway is added to a serum-free medium within 5 days, morepreferably within 3 days, from the start of the suspension culture, mostpreferably simultaneously with the start of the suspension culture. Inaddition, suspension culture is performed up to day 18, more preferablyday 12, from the start of the suspension culture with the addition of asubstance inhibiting the Wnt signal pathway.

The culture conditions such as culture temperature, and CO₂concentration in the first step can be appropriately determined. Whilethe culture temperature is not particularly limited, it is, for example,about 30 to 40° C., preferably about 37° C. The CO₂ concentration is,for example, about 1 to 10%, preferably around 5%.

The concentration of the pluripotent stem cells in the first step can bedetermined as appropriate by those of ordinary skill in the art to formaggregates of pluripotent stem cells more uniformly and efficiently. Theconcentration of the pluripotent stem cells when forming aggregates isnot particularly limited as long as it permits formation of uniformaggregates of stem cells. For example, when human ES cells are subjectedto suspension culture using a 96 well microwell plate, a liquid preparedto about 1×10³ to about 5×10⁴ cells, preferably about 3×10³ to about3×10⁴ cells, more preferably about 5×10³ to about 2×10⁴ cells, mostpreferably around 9×10³ cells, per well is added, and the plate is leftstanding to form aggregates.

The time of suspension culture necessary for forming aggregates can bedetermined as appropriate according to the pluripotent stem cell to beused, as long as the cells can be aggregated rapidly. To form uniformaggregates, it is desirably as short as possible. For example, in thecase of human ES cells, aggregates are desirably formed preferablywithin 24 hr, more preferably within 12 hr. The time for aggregateformation can be appropriately adjusted by those of ordinary skill inthe art by controlling the tools for aggregating the cells,centrifugation conditions and so on.

Those of ordinary skill in the art can determine whether aggregates ofpluripotent stem cells have been formed, based on the size and cellnumber of aggregates, macroscopic morphology, microscopic morphology bytissue staining analysis and uniformity thereof, expression ofdifferentiation and undifferentiation markers and uniformity thereof,control of expression of differentiation marker and synchronism thereof,reproducibility of differentiation efficiency between aggregates, and soon.

The basement membrane preparation to be used in the second step refersto one containing basement membrane-constituting components having afunction to control cell morphology, differentiation, growth, motility,expression of function and so on which are similar to those ofepithelial cell, when intended cells capable of forming a basementmembrane are plated thereon and cultured. Here, the “basement membraneconstituting component” refers to an extracellular matrix molecule inthe morphology of a thin membrane present between epithelial cell layerand interstitial cell layer and so on in animal tissues. A basementmembrane preparation can be produced by, for example, removing cellscapable of forming a basement membrane, which adhere onto a support viaa basement membrane, with a solution capable of dissolving the lipid ofthe cells, an alkali solution and so on. Examples of preferable basementmembrane preparation include products commercially available as basementmembrane components (e.g., Matrigel (hereinafter, sometimes referred toas Matrigel)), and extracellular matrix molecules known as basementmembrane components (e.g., laminin, type IV collagen, heparan sulfateproteoglycan, entactin and so on).

Matrigel is a product prepared from a basement membrane derived fromEngelbreth Holm Swarn (EHS) mouse sarcoma. The main component ofMatrigel is type IV collagen, laminin, heparan sulfate proteoglycan, andentactin. In addition to these, TGF-β, fibroblast growth factor (FGF),tissue plasminogen activator, and a growth factor naturally produced byEHS tumor are contained. The “growth factor reduced (GFR) product” ofMatrigel has a lower growth factor concentration than common Matrigel.In the present invention, GFR product is preferably used.

While the concentration of the basement membrane preparation to be addedto a serum-free medium for the suspension culture in the second step isnot particularly limited as long as the epithelial structure of theneural tissue (e.g., retinal tissue) is stably maintained, for example,it is preferably 1/20 to 1/200 volume, more preferably around 1/100volume, of the culture medium when Martigel is used. While basementmembrane preparation may already have been added to the medium when theculture of stem cell is started, it is preferably added to theserum-free medium within 5 days, more preferably within 2 days, from thestart of the suspension culture.

As the serum-free medium to be used in the second step, the serum-freemedium used in the first step may be directly used, or may be replacedwith a fresh serum-free medium.

When the serum-free medium used in the first step is directly used forthis step, the “basement membrane preparation” can be added to themedium.

The culture conditions such as culture temperature, and CO₂concentration in the second step can be appropriately determined. Whilethe culture temperature is not particularly limited, it is, for example,about 30 to 40° C., preferably around 37° C. The CO₂ concentration is,for example, about 1 to 10%, preferably around 5%.

As the serum-containing medium to be used in the third step, may be usedthe serum-free medium used in the culture of the second step to which aserum is directly added, or one replaced with a fresh serum-containingmedium.

The serum is added on or after day 7, more preferably on or after day 9,most preferably on day 12, from the start of the suspension culture. Theconcentration of the serum to be added is about 1 to 30%, preferablyabout 3 to 20%, more preferably around 10%.

In the third step, the production efficiency of retinal tissue can beincreased by adding a substance acting on the Shh signal pathway inaddition to the serum.

The substance acting on the Shh signal pathway is not particularlylimited as long as it can enhance signal transduction mediated by Shh.Examples of the substance acting on the Shh signal pathway includeproteins belonging to the Hedgehog family (e.g., Shh), Shh receptor, Shhreceptor agonist, Purmorphamine, SAG and so on.

The concentration of the substance acting on the Shh signal pathway usedin this step is, for example, in the case of common substance acting onthe Shh signal pathway such as SAG, about 0.1 nM to 10 μM, preferablyabout 10 nM to 1 μM, more preferably around 100 nM.

The retinal tissue thus produced is present to cover the surface of theaggregate. Whether a retinal tissue is produced can be confirmed byimmunostaining method etc.

For example, the aggregate cultured in the third step is subjected tosuspension culture in a serum-containing medium. Examples of the cellculture vessel to be used for suspension culture include those mentionedabove. The culture conditions such as culture temperature, CO₂concentration, and O₂ concentration of the suspension culture can beappropriately determined. While the culture temperature is notparticularly limited, it is, for example, about 30 to 40° C., preferablyabout 37° C. The CO₂ concentration is, for example, about 1 to 10%,preferably about 5%. The 0₂ concentration is, for example, 20 to 70%,preferably 20 to 60%, more preferably 30 to 50%. While the cultureperiod is not particularly limited, it is generally not less than 48 hr,preferably not less than 7 days.

After completion of the suspension culture, the aggregates are fixedwith a fixative such as para-formaldehyde solution and so on, and acryosection is prepared. The obtained cryosection is immunostained, andformation of a layer structure of retinal tissue is confirmed. Sincerespective layers of a retinal tissue are composed of different retinalprogenitor cells (photoreceptor, horizontal cell, bipolar cell, amacrinecell, retinal ganglion cell), formation of a layer structure can beconfirmed by immunostaining using antibodies against the aforementionedmarkers expressed in these cells.

The “proportion of Chx10 positive cells” in a retinal tissue containedin a cell aggregate produced as mentioned above can be examined by, forexample, the following method.

-   (1) First, a cryosection of “a cell aggregate comprising a retinal    tissue” is prepared.-   (2) Then, immunostaining of Rax protein or, when a gene recombinant    cell obtained by altering a Rax gene expressing cell to express a    fluorescence protein such as GFP is used, the expression of the    aforementioned fluorescence protein, is observed using a    fluorescence microscope and so on, whereby a retinal tissue region    expressing Rax gene is specified.-   (3) Using the same section as the cryosection wherein the retinal    tissue region expressing Rax gene has been specified or an adjacent    section as a sample, the nucleus is stained with a nuclear staining    reagent such as Dapi. Then, the number of stained nuclei in the    above-specified retinal tissue region expressing Rax gene is    counted, whereby the number of the cells in the retinal tissue    region is measured.-   (4) Using the same section as the cryosection wherein the retinal    tissue region expressing Rax gene has been specified or an adjacent    section as a sample, Chx10 protein is immunostained. The number of    Chx10 positive cells in the above-specified retinal tissue region is    counted.-   (5) Based on each number of nuclei measured in the    above-mentioned (3) and (4), the number of nuclei in Chx10 positive    cells is divided by the number of nuclei in the Chx10 positive cells    in the above-specified retinal tissue region, whereby the    “proportion of Chx10 positive cells” is calculated.

In the production method of the present invention, firstly, a cellaggregate comprising a retinal tissue in which Chx10 positive cells arepresent in a proportion of 20% or more of the tissue is cultured in aserum-free medium or serum-containing medium each containing a substanceacting on the Wnt signal pathway for only a period before the appearanceof a RPE65 gene expressing cell.

As a preferable culture here, suspension culture can be mentioned. As apreferable medium, a serum-free medium can be mentioned.

The culture conditions such as culture temperature, CO₂ concentrationcan be appropriately set. The culture temperature is, for example, inthe range of about 30° C. to about 40° C., preferably, for example,around 37° C. The CO₂ concentration is, for example, in the range ofabout 1% to about 10%, preferably, for example, around 5%.

The substance acting on the Wnt signal pathway to be contained in aserum-free medium or serum-containing medium when the above-mentioned“cell aggregate comprising a retinal tissue” is cultured in the mediumis not particularly limited as long as it can enhance signaltransduction mediated by Wnt. Specific examples of the substance actingon the Wnt signal pathway include protein belonging to Wnt family, Wntreceptor, Wnt receptor agonist, GSK3β inhibitor (e.g.,6-Bromoindirubin-3′-oxime (BIO), CHIR99021, Kenpaullone) and so on.

The concentration of the substance acting on the Wnt signal pathway tobe contained in a serum-free medium or serum-containing medium in thecase of a common substance acting on the Wnt signal pathway such asCHIR99021 is, for example, in the range of about 0.1 μM to 100 μM,preferably, for example, in the range of about 1 μM to 30 μM, morepreferably, for example, around 3 μM.

“Culturing for only a period before the appearance of a RPE65 geneexpressing cell” in the production method of the present invention meansculturing in the whole or a part of the period before the appearance ofa RPE65 gene expressing cell. That is, culturing in the whole or a partof the period (any period) during which the “cell aggregate comprising aretinal tissue” in the culture system is constituted by cells that donot substantially express RPE65 gene suffices. By employing suchculturing, a cell aggregate in which a RPE65 gene expressing cell doesnot appear can be obtained.

To determine such particular period, the “cell aggregate comprising aretinal tissue” is used as a sample, and the presence or absence ofexpression of RPE65 gene contained in the sample only needs to bemeasured by a general genetic engineering method. Specifically, forexample, as described in the below-mentioned Examples, the presence orabsence of expression of RPE65 gene or the level thereof can be examinedby subjecting a cryosection of the aforementioned “cell aggregatecomprising a retinal tissue” to an immunostaining method using anantibody against RPE65 protein.

A preferable “period before the appearance of a RPE65 gene expressingcell” is, for example, a period during which Chx10 positive cells arepresent in the retinal tissue in a proportion of within 50% to 1% of thetissue. In this case, the obtained “cell aggregate in which a RPE65 geneexpressing cell does not appear” is a cell aggregate in which Chx10positive cells are present in the retinal tissue in a proportion ofwithin 50% to 1% of the tissue.

While the number of days of the “period before the appearance of a RPE65gene expressing cell” varies depending on the kind of the substanceacting on the Wnt signal pathway, the kind of the serum-free medium orserum-containing medium, other culture conditions and so on, it is, forexample, within 5 days. The aforementioned period is preferably, forexample, within 4 days, more preferably, for example, 2 days to 3 days.

Then the “cell aggregate in which a RPE65 gene expressing cell does notappear” obtained by culturing as mentioned above is cultured in aserum-free medium or serum-containing medium not containing a substanceacting on the Wnt signal pathway.

A preferable culture here is, for example, suspension culture.

Examples of a preferable culture time include a time for culturing untilthe proportion of the Chx10 positive cells present in the retinal tissuereaches 50% or more of the tissue.

The culture conditions such as culture temperature, CO₂ concentrationcan be appropriately set. The culture temperature is, for example, inthe range of about 30° C. to about 40° C., preferably, for example,around 37° C. In addition, the CO₂ concentration is, for example, in therange of about 1% to about 10%, preferably, for example, around 5%.

While the number of the above-mentioned culture days until “a cellaggregate comprising a ciliary marginal zone-like structure” is obtainedvaries depending on the kind of the serum-free medium orserum-containing medium, other culture conditions and so on, it is, forexample, within 100 days. The aforementioned number of culture days ispreferably, for example, 20 days to 70 days, more preferably, forexample, 30 days to 60 days.

In the “cell aggregate comprising a ciliary marginal zone-likestructure” thus produced, retinal pigment epithelium and retinal tissue(specifically, neural retina) are present adjacent to the ciliarymarginal zone-like structure in the same cell aggregate. The structurecan be easily confirmed by microscopic observation and so on.

A highly pure retinal tissue (specifically, neural retina) can beprepared by physically cutting out a retinal tissue (specifically,neural retina) from the aforementioned “cell aggregate comprising aciliary marginal zone-like structure” with tweezers etc. A highly pureretinal tissue (specifically, neural retina) can be further continuouslycultured (specifically, long-term culture for, for example, 60 days orlonger) while maintaining the good tissue structure it has. The cultureconditions such as culture temperature, CO₂ concentration, O₂concentration can be those generally used for tissue culture. In thiscase, culture may be performed in the presence of a serum, a knowngrowth factor, an additive and a chemical substance that promote thegrowth, and so on. Examples of the known growth factor include EGF, FGFand so on. Examples of the additive that promotes the growth include N2supplement (Invitrogen), B27 supplement (Invitrogen) and so on.

The present invention also includes use of a cell aggregate comprising aciliary marginal zone-like structure produced by the production methodof the present invention as a reagent for the evaluation of the toxicityor drug efficacy, use of a cell aggregate comprising a ciliary marginalzone-like structure produced by the production method of the presentinvention as a biological material for transplantation and so on.

<Use of Cell Aggregate Comprising Ciliary Marginal Zone-Like Structureas Reagent for Evaluating Toxicity or Drug Efficacy>

The cell aggregate comprising a ciliary marginal zone-like structureproduced by the production method of the present invention can be usedfor screening for a therapeutic drug for a disease caused by a disorderof retinal cell, a material for the study of diseases or a drugdiscovery material. It is also utilizable for the evaluation of thetoxicity or drug efficacy of a chemical substance and so on, as well asstudy of toxicity such as phototoxicity, neurotoxicity and so on,toxicity test and so on.

<Use of Cell Aggregate Comprising Ciliary Marginal Zone-Like Structureas Biological Material for Transplantation>

The cell aggregate comprising a ciliary marginal zone-like structureproduced by the production method of the present invention can be usedas a biological material for transplantation used for supplementing adisordered tissue itself in a cell damage state (e.g., used fortransplantation operation) and so on.

EXAMPLES

The present invention is explained in more detail in the following byreferring to Examples, which are not to be construed as limitative.

Example 1 Production Example of Cell Aggregate Containing Retinal TissueUsing Human ES Cell—1

RAX::GFP knock-in human ES cells (derived from KhES-1; Nakano, T. et al.Cell Stem Cell 2012, 10(6), 771-785) were cultured according to themethods described in “Ueno, M. et al. PNAS 2006, 103(25), 9554-9559” and“Watanabe, K. et al. Nat Biotech 2007, 25, 681-686”. As the medium,DMEM/F12 medium (Invitrogen) added with 20% KSR (Knockout SerumReplacement; Invitrogen), 0.1 mM 2-mercaptoethanol, 1 mM pyruvic acidand 5 to 10 ng/ml bFGF was used. The aforementioned cultured ES cellswere singly dispersed in 0.25% trypsin-EDTA (Invitrogen), and the singlydispersed ES cells were floated in a 100 μl serum-free medium to 9×10³cells per well of a non-cell adhesive 96-well culture plate (SUMILONspheroid plate, SUMITOMO BAKELITE CO., LTD.), and suspension-cultured at37° C., 5% CO₂. The serum-free medium used then was a serum-free mediumobtained by adding 20% KSR, 0.1 mM 2-mercaptoethanol, 1 mM pyruvic acid,20 μM Y27632 and Wnt signal pathway inhibitory substance (3 μM IWR1e) toG-MEM medium. During the suspension culture, GFR Matrigel (Invitrogen)in an amount of 1/100 per volume was added from day 2 from the start ofthe suspension culture. A fetal bovine serum in an amount of 1/10 pervolume and a substance acting on the Shh signal pathway (100 nM SAG)were added on day 12 from the start of the suspension culture, and thesuspension culture was performed for total 18 days.

The thus-produced cell aggregate was fixed with 4% para-formaldehyde toproduce a cryosection. The prepared cryosection was subjected tofluorescence microscope observation of a GFP fluorescence image (FIG. 1) and immunostaining with Chx10 which is one of neural retinalprogenitor cell markers (FIG. 2 ). In the retinal tissue contained inthe cell aggregate produced as mentioned above, Chx10 positive cellswere found in about 40% of the tissue (see FIG. 2 ).

Example 2 Production Example of Cell Aggregate Containing Retinal TissueUsing Human ES Cell—2

By a method similar to that in Example 1, suspension culture wasperformed for a longer time. As a result, on day from the start of thesuspension culture, a cell aggregate containing a retinal tissue inwhich Chx10 positive cells were found in about 80% or about 90% of thetissue was also obtained.

Example 3 Culture of Cell Aggregate Containing Retinal Tissue inSerum-Free Medium Containing Substance Acting on Wnt Signal Pathway

A cell aggregate containing a retinal tissue on day 18 from the start ofthe suspension culture, which was produced by the method described inExample 1, was suspension-cultured in a serum-free medium containing asubstance acting on the Wnt signal pathway (3 μM CHIR99021) for 3 days.

The obtained cell aggregate was fixed with 4% para-formaldehyde toprepare a cryosection. The prepared cryosection was subjected tofluorescence microscopic observation of a GFP fluorescence image (FIG. 3) and immunostaining with Chx10 which is one of the neural retinalprogenitor cell markers (FIG. 4 ). In the retinal tissue contained inthe aforementioned cell aggregate, which had been suspension-cultured ina serum-free medium containing a substance acting on the Wnt signalpathway for 3 days, Chx10 positive cells were found in only about 3% ofthe tissue and a clear decrease in the proportion of the Chx10 positivecells was observed (see FIG. 4 ). At that time, a RPE65 gene expressingcell did not appear in the aforementioned cell aggregate.

Example 4 Suspension Culture of “Cell Aggregate in Which a RPE65 GeneExpressing Cell Does Not Appear” in Serum-Containing Medium NotContaining a Substance Acting on Wnt Signal Pathway—1

The cell aggregate (the proportion of Chx10 positive cells: about 3%) onday 21 from the start of the suspension culture (total days of theabove-mentioned “day 18” and “day 3”), which was produced by the methodsdescribed in Example 1 and Example 3, was further suspension-cultured ina serum-containing medium (containing DMEM/F12, 10% fetal bovine serum,N2 supplement, 0.5 μM retinoic acid etc.) not containing a substanceacting on the Wnt signal pathway under 40% O₂ condition for 39 days.

The obtained cell aggregate was fixed with 4% para-formaldehyde toprepare a cryosection. The prepared cryosection was subjected tofluorescence microscopic observation of a GFP fluorescence image (FIG. 5) and immunostaining with Rdh10 which is one of the ciliary marginalzone markers (FIG. 6 ). In the manner mentioned above, it could beconfirmed in the cell aggregate after suspension culture for 39 days ina serum-containing medium not containing a substance acting on the Wntsignal pathway that Rdh10 positive cells (that is, cells expressingRdh10 gene which is a marker gene of ciliary marginal zone-likestructure) were present as a nearly-uniform group region in the tissuepresent in the boundary region of retinal tissue (specifically, neuralretina) and retinal pigment epithelium, and a cell aggregate containinga ciliary marginal zone-like structure was produced with high efficiency(see FIG. 6 ).

Example 5 Suspension Culture of “Cell Aggregate in Which a RPE65 GeneExpressing Cell Does Not Appear” in Serum-Containing Medium NotContaining a Substance Acting on Wnt Signal Pathway—2)

The cell aggregate (existence ratio of Chx10 positive cells: about 3%)on day 21 from the start of the suspension culture (total days of theabove-mentioned “day 18” and “day 3”), which was produced by the methodsdescribed in Example 1 and Example 3, was further suspension-cultured inthe same manner as in Example 4 in a serum-containing medium (containingDMEM/F12, 10% fetal bovine serum, N2 supplement, 0.5 μM retinoic acidetc.) not containing a substance acting on the Wnt signal pathway under40% O₂ condition for further 50 days.

The obtained cell aggregate was fixed with 4% para-formaldehyde toprepare a cryosection. The prepared cryosection was subjected tofluorescence microscopic observation of a GFP fluorescence image (FIG. 7) and immunostaining with Rdh10 (FIG. 8 ) or Otx1 (FIG. 9 ) which is oneof the ciliary marginal zone markers. In the manner mentioned above, inthe cell aggregate after suspension culture for 50 days in aserum-containing medium not containing a substance acting on the Wntsignal pathway, Rdh10 positive cells (that is, cells expressing Rdh10gene which is a marker gene of ciliary marginal zone-like structure)were present as a nearly-uniform group region in the tissue present inthe boundary region of retinal tissue (specifically, neural retina) andretinal pigment epithelium (see FIG. 8 ), and Otx1 positive cells (thatis, cells expressing Otx1 gene which is a marker gene of ciliarymarginal zone-like structure) were also similarly present (see FIG. 9 ).From these results, it could be confirmed that a cell aggregatecontaining a ciliary marginal zone-like structure was produced with highefficiency by the production method.

Example 6 Analysis of Proliferative Capacity of Cell AggregateContaining Ciliary Marginal Zone-Like Structure—1

The cell aggregate on day 18 from the start of the suspension culture,which was produced by the method described in Example 1, wassuspension-cultured for 3 days in a serum-free medium containing asubstance acting on the Wnt signal pathway, and then suspension-culturedin the same manner as in Example 4 and Example 5 for 46 days in aserum-containing medium not containing a substance acting on the Wntsignal pathway. Thereafter, the cell aggregate was cultured in thepresence of BrdU for 1 day to label the grown cells, then in the absenceof BrdU for 13 days, and in the presence of EdU (Invitrogen) for 1 day.The obtained cell aggregate was fixed with 4% para-formaldehyde toprepare a cryosection. The prepared cryosection was subjected toimmunofluorescence staining with anti-Ki67 antibody (FIG. 10 , leftFigure) or anti-BrdU antibody (FIG. 10 , right Figure), or EdU colordevelopment reaction (FIG. 10 , middle Figure).

As a result, it was found that the ciliary marginal zone-like structurewas a Ki67 positive growing cell in the cell aggregate after suspensionculture for 46 days in a serum-containing medium not containing asubstance acting on the Wnt signal pathway in the manner mentioned above(FIG. 10 , left Figure, shown by arrow). It was found that 90% or moreof the growing cells can be labeled by EdU uptake for 1 day, since 90%or more of Ki67 positive cells are EdU-positive (FIG. 10 , middleFigure, arrow). On the other hand, since Ki67 positive cells in theciliary marginal zone-like structure showed a weak 30 BrdU signal (FIG.10 , right Figure, arrow), it is assumed that the aforementioned Ki67positive cells continued to grow for 14 days after BrdU labeling anddiluted BrdU uptaken into DNA. From these results, it was found that theciliary marginal zone-like structure in the cell aggregate cultured asmentioned above is the progress zone.

Example 7 Analysis of Proliferative Capacity of Cell AggregateContaining Ciliary Marginal Zone-Like Structure—2

The cell aggregate on day 18 from the start of the suspension culture,which was produced by the method described in Example 1, wassuspension-cultured for 3 days in a serum-free medium containing asubstance acting on the Wnt signal pathway, and then suspension-culturedin the same manner as in Example 4, Example 5 and Example 6 for 46 daysin a serum-containing medium not containing a substance acting on theWnt signal pathway. Thereafter, the cell aggregate was cultured in thepresence of BrdU for 1 day to label the grown cells, then in the absenceof BrdU for 13 days, in the presence of EdU (Invitrogen) for 1 day, andfurther in the absence of EdU for 13 days. A cryosection of the obtainedcell aggregate was produced, and subjected to immunofluorescencestaining with anti-Ki67 antibody (FIG. 11 , left Figure), anti-BrdUantibody (FIG. 11 , right Figure) or anti-Rdh10 antibody (FIG. 11 ,bottom Figure), or EdU color development reaction (FIG. 11 , middleFigure).

As a result, it was found that the Rdh10-positive ciliary marginalzone-like structure (FIG. 11 , bottom Figure, shown by 25 arrow) wasKi67 positive (FIG. 11 , left Figure) in the cell aggregate aftersuspension culture for 46 days in a serum-containing medium notcontaining a substance acting on the Wnt signal pathway in the mannermentioned above. The aforementioned Ki67 positive cell in the ciliarymarginal zone-like structure was found to show weak EdU (FIG. 11 ,middle Figure) and BrdU (FIG. 11 , right Figure) signals. Therefore, itwas assumed that the aforementioned Ki67 positive cell in theaforementioned ciliary marginal zone-like structure continued to growfor 27 days and diluted EdU and BrdU uptaken into DNA. From theseresults, it was found that the ciliary marginal zone-like structure inthe cell aggregate cultured as mentioned above is the progress zone.

Example 8 Analysis of Morphology of Neural Retina in Cell AggregateContaining Ciliary Marginal Zone-Like Structure

The cell aggregate on day 18 from the start of the suspension culture,which was produced by the method described in Example 1, wassuspension-cultured for 3 days in a serum-free medium containing asubstance acting on the Wnt signal pathway and, in the same manner as inExample 4, Example 5, Example 6 and Example 7, suspension-cultured for75 days in a serum-containing medium not containing a substance actingon the Wnt signal pathway, and the obtained cell aggregate was analyzed.As one embodiment of the cell aggregate, a phase contrast image of thecell aggregate without a ciliary marginal zone-like structure (CMZ) (A,left line, the upper panel), a GFP fluorescence image of Crxgene-expressing cells of the cell aggregate without a ciliary marginalzone-like structure (A, left row, lower panel), a phase contrast imageof the cell aggregate containing a ciliary marginal zone-like structure(A, right row, upper panel), and a GFP fluorescence image of Crxgene-expressing cells of the cell aggregate containing a ciliarymarginal zone-like structure (A, right row, lower panel) are shown. Inthe GFP fluorescence image of Crx gene-expressing cell in a cellaggregate containing a ciliary marginal zone-like structure (A, rightrow, lower panel), a continuous neural retina having a layer structurewas found to exist adjacent to the ciliary marginal zone-like structurein the lower right part (shown by arrow).

The proportion of cell aggregates containing a continuous neural retinahaving a layer structure in not less than 10% of the cell aggregatecircumference, in the cell aggregates without a ciliary marginalzone-like structure (CMZ−) and the cell aggregates with a ciliarymarginal zone-like structure (CMZ+) was measured by using the morphologyof the Crx gene-expressing cell as an index (FIG. 12B). As a result, itwas found that the cell aggregate with a ciliary marginal zone-likestructure (CMZ+) has a higher ratio of cell aggregates containing acontinuous neural retina having a layer structure, as compared to thecell aggregates without a ciliary marginal zone-like structure (CMZ−).

Industrial Applicability

According to the production method of the present invention, a ciliarymarginal zone-like structure can be produced with high efficiency. In acell aggregate comprising a ciliary marginal zone-like structure, whichis produced by the production method of the present invention, theciliary marginal zone-like structure functions as a progress zone, and acontinuous neural retina having a layer structure can be formed adjacentto the ciliary marginal zone-like structure with high frequency.

This application is based on patent application No. 2012-130521 filed inJapan (filing date: Jun. 8, 2012), the contents of which areincorporated in full herein. All publications, patents and patentpublications cited in the Specification are hereby incorporated byreference.

The invention claimed is:
 1. A method for producing an artificial cellaggregate having a ciliary marginal zone-like structure with Rdh10positive cells, a retinal pigment epithelium and a neural retinacomprising: (a) preparing a first cell aggregate from a pluripotent stemcell, (b) culturing the cell aggregate to form a retinal tissuecomprising at least 20% of cells positive for Chx10, (c) culturing thecell aggregate having the retinal tissue of step (b) in a serum-freemedium containing a substance that enhances signal transduction mediatedby Wnt for a period before the appearance of RPE65 expressing cells,thereby obtaining a cell aggregate having a retinal tissue with adecreased proportion of Chx10 positive cells that is about 3% or less ofthe tissue, (d) culturing the cell aggregate having the retinal tissuewith the decreased proportion of Chx10 positive cells of step (c)without the substance that enhances signal transduction mediated by Wntin a serum-free medium containing a serum replacement or aserum-containing medium to induce the formation of a ciliary marginalzone-like structure with Rdh10 positive cells in the retinal tissue, and(e) obtaining the cell aggregate having the ciliary marginal zone-likestructure formed in the retinal tissue, wherein the ciliary marginalzone-like structure comprises Rdh10 positive cells, and wherein aretinal pigment epithelium and a neural retina are present adjacent tothe ciliary marginal zone-like structure on the surface of the cellaggregate.
 2. The method according to claim 1, wherein the decreasedproportion of Chx10 positive cells in step (c) is within 3% to 1% of thetissue.
 3. The method according to claim 2, wherein the culturing instep (d) is until the proportion of Chx10 positive cells present in theretinal tissue reaches 50% or more of the tissue.
 4. The methodaccording to claim 1, wherein the pluripotent stem cell is a humanpluripotent stem cell.
 5. The method according to claim 1, wherein theculturing of the cell aggregate in step (b) is to form a retinal tissuecomprising at least 40% of cells positive for Chx10.
 6. The methodaccording to claim 1, wherein the culturing of the cell aggregate instep (c) is for 2 to 5 days.
 7. An artificial cell aggregate produced bythe method according to claim 1, the cell aggregate comprising a ciliarymarginal zone-like structure, a retinal pigment epithelium and a neuralretina, wherein the retinal pigment epithelium and the neural retina arepresent adjacent to the ciliary marginal zone-like structure on thesurface of the cell aggregate, wherein the neural retina is acontinuous, stratified neural retina with Crx positive cells, andwherein the ciliary marginal zone-like structure comprises Rdh10positive cells.
 8. An artificial cell aggregate comprising a ciliarymarginal zone-like structure, a retinal pigment epithelium and a neuralretina, wherein the retinal pigment epithelium and the neural retina arepresent adjacent to the ciliary marginal zone-like structure on thesurface of the cell aggregate, wherein the neural retina is acontinuous, stratified neural retina with Crx positive cells, andwherein the ciliary marginal zone-like structure comprises Rdh10positive cells.
 9. The artificial cell aggregate according to claim 8,wherein the neural retina is present in not less than 10% of thecircumference of the cell aggregate.
 10. The method for producing aneural retina comprising cutting out the neural retina from theartificial cell aggregate according to claim 8, thereby obtaining apurified neural retina.
 11. The artificial cell aggregate according toclaim 8, wherein the aggregate is formed from a human pluripotent stemcell.
 12. The artificial cell aggregate according to claim 8, whereinthe ciliary marginal zone-like structure comprises Rdh10 positive cells,Otx1 positive cells and Ki67 positive cells.
 13. The artificial cellaggregate according to claim 8, wherein the retinal pigment epitheliumis pigmented.